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| author | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
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| committer | Laurent Bercot <ska-skaware@skarnet.org> | 2014-12-05 22:26:11 +0000 |
| commit | 90b12bd71bb9fc79a4640b9112c13ef529d0196a (patch) | |
| tree | 523b3f4ee2969e7a729bab2ba749c4b924ae62af /doc/s6-log.html | |
| download | s6-90b12bd71bb9fc79a4640b9112c13ef529d0196a.tar.xz | |
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diff --git a/doc/s6-log.html b/doc/s6-log.html new file mode 100644 index 0000000..0a525d6 --- /dev/null +++ b/doc/s6-log.html @@ -0,0 +1,508 @@ +<html> + <head> + <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" /> + <meta http-equiv="Content-Language" content="en" /> + <title>s6: the s6-log program</title> + <meta name="Description" content="s6: the s6-log program" /> + <meta name="Keywords" content="s6 command s6-log log logger logging multilog" /> + <!-- <link rel="stylesheet" type="text/css" href="http://skarnet.org/default.css" /> --> + </head> +<body> + +<p> +<a href="index.html">s6</a><br /> +<a href="http://skarnet.org/software/">Software</a><br /> +<a href="http://skarnet.org/">skarnet.org</a> +</p> + +<h1> The s6-log program </h1> + +<p> +s6-log is a reliable logging program with automated log rotation, similar to +daemontools' <a href="http://cr.yp.to/daemontools/multilog.html">multilog</a>, +with full POSIX regular expression support. +</p> + +<h2> Interface </h2> + +<pre> + s6-log [ -q | -v ] [ -b ] [ -p ] [ -t ] [ -e ] <em>logging script</em> +</pre> + +<p> +s6-log reads and compiles <em>logging script</em> to an internal +form. Then it reads its standard input, line by line, and performs actions +on it, following the script it is given. It does its best to ensure there +is <em>never any log loss</em>. It exits cleanly when stdin closes or when +it receives SIGTERM. +</p> + +<h2> Options </h2> + +<ul> + <li> <tt>-b</tt> : blocking mode. With this option, s6-log stops +reading its standard input while it has unflushed buffers. This ensures that +every log line has been fully processed before reading the next one; this is also +<a href="http://cr.yp.to/daemontools/multilog.html">multilog</a>'s behaviour. +By default, s6-log keeps reading from stdin even if its buffers still +contain data. <tt>-b</tt> is safer, but may slow down your service; the default +is faster, but may lead to unbound memory use if you have a lot of output to +write to a slow file system. </li> + <li> <tt>-p</tt> : protect. Do not exit on receipt of a SIGTERM; only +exit when reading EOF on stdin. </li> + <li> <tt>-t</tt> : timestamp. Prepends every log line that is written to +a logging directory with a +<a href="http://skarnet.org/software/skalibs/libstddjb/tai.html">TAI64N</a> +timestamp. </li> + <li> <tt>-e</tt> : timestamp alerts. Prepends every "alert" line with a +TAI64N timestamp. </li> + <li> <tt>-q | -v</tt> : quiet | verbose. Decreases | increases s6-log's +verbosity, i.e. which messages are sent to stderr. The default verbosity is 1. +Currently supported verbosity levels: + <ul> + <li> 0: only write alerts and fatal errors </li> + <li> 1: write alerts, warnings and fatal errors </li> + </ul> </li> +</ul> + +<h2> Logdirs </h2> + +<p> +A <em>logdir</em> (<em>logging directory</em>) is a place where logs are +stored. s6-log can be scripted to write into one or more logdirs. +</p> + +<p> + A logdir may contain the following files: +</p> + +<ul> + <li> <tt>lock</tt>: this file is locked by s6-log at start, +to make sure only one instance is running at the same time. </li> + <li> <tt>current</tt>: the file where selected log lines are appended. +If <tt>current</tt> has the executable-by-user flag, it means that no +s6-log process is currently writing to it and the previous +s6-log process managed to cleanly finalize it. If it does not, +either a s6-log process is writing to it or the previous one +has been interrupted without finalizing it. </li> + <li> <tt>state</tt>: last processor's output, see below. </li> + <li> <tt>previous</tt>: a rotation is happening in that logdir. </li> + <li> <tt>processed</tt>, <tt>newstate</tt>: a rotation is happening +in that logdir and the processor script is running. </li> + <li> timestamped files: those files are @<em>timestamp</em>.s or +@<em>timestamp</em>.u and are old log files that have been processed +and rotated (if they're ending in .s) or that were the <tt>current</tt> +file when s6-log got interrupted (if they're ending in .u), in which +case they have not been processed. </li> +</ul> + +<h3> Rotation </h3> + +<p> + In a logdir, selected lines are appended to the <tt>current</tt> file. +When <tt>current</tt> becomes too big, a <em>rotation</em> happens. +The <tt>current</tt> file will be possibly <em>processed</em>, then +it will become an <em>archived</em> log file named +@<em>timestamp</em>.s, where <em>timestamp</em>, a +<a href="http://www.skarnet.org/software/skalibs/libstddjb/tai.html">TAI64N</a> +timestamp, is the absolute time of the rotation. If there are too +many archived log files in the logdir, the older ones are then +suppressed. Logging then resumes, to a brand new <tt>current</tt> +file. +</p> + +<p> + You can use this mechanism to ensure that your logs never fill up +the available disk space, for instance: something that neither +syslogd, nor syslog-ng, nor rsyslog offers. +</p> + +<h3> Processors </h3> + +<p> + A <em>processor</em> script can be set for every logdir. When a rotation +occurs, <tt>current</tt> (which has then been renamed <tt>previous</tt>) is +fed to <em>processor</em>'s stdin, and <em>processor</em>'s stdout is saved +and archived. <em>processor</em> can also read the <tt>state</tt> file +on its fd 4; what it writes to its fd 5 will be saved as the next +<tt>state</tt> file, for the next rotation. +</p> + +<p> + Processors should not background themselves: s6-log considers the +processing done when its <em>processor</em> direct child dies. +Processors should exit 0 on success and nonzero on failure; if a +processor fails, s6-log will try it again after some +<em>cooldown</em> time. +</p> + +<p> + Processors make s6-log Turing-complete by allowing you to use any +external program to handle log files that are going to be archived. +</p> + +<h2> Logging script syntax </h2> + +<p> + When starting up, s6-log reads its arguments one by one; this +argument sequence, or <em>directive sequence</em>, forms a +<em>logging script</em> which tells s6-log what to log, where, and how. +</p> + +<p> + Every directive can be a <em>selection</em> directive, a <em>control</em> +directive or an <em>action</em> directive. A valid logging script always +contains at least one action directive; every action directive can be +preceded by zero or more selection or control directives. s6-log will exit 100 +if the script is invalid. If it can process the script but the last directive +is not an action directive, s6-log will emit a warning. +</p> + +<h3> Selection directives </h3> + +<p> + These directives tell s6-log whether to select or deselect lines it reads +from stdin; actions will only happen on selected lines. By default, every +line is selected. +</p> + +<ul> + <li> <strong>+<em>regexp</em></strong>: select yet-unselected lines that match <em>regexp</em>, which must be a +<a href="http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_04">POSIX +Extended Regular Expression</a>. </li> + <li> <strong>-<em>regexp</em></strong>: deselect yet-selected lines that match <em>regexp</em>, which must be a +POSIX Extended Regular Expression. </li> + <li> <strong>f</strong>: select exactly lines that have not yet been acted +upon (i.e. that were always deselected when the script encountered an action +directive). </li> +</ul> + +<h3> Control directives </h3> + +<p> + These directives tune s6-log's behaviour for the next actions. +</p> + +<ul> + <li> <strong>n<em>number</em></strong>: next logdirs will contain up to +<em>number</em> archived log files. If there are more, the oldest archived +log files will be suppressed, only the latest <em>number</em> will be kept. +By default, <em>number</em> is 10. </li> + <li> <strong>s<em>filesize</em></strong>: next rotations will occur when +<tt>current</tt> log files approach <em>filesize</em> bytes. By default, +<em>filesize</em> is 99999; it cannot be set lower than 4096 or +higher than 16777215. </li> + <li> <strong>S<em>totalsize</em></strong>: next logdirs will contain up +to <em>totalsize</em> bytes of archived (and maybe processed) log files. If +archived log files take more space than that, the older ones are suppressed +until the total size fits. A <em>totalsize</em> of zero means no such limit; +use <strong>n0</strong> instead if you don't want any archived log files. By +default, <em>totalsize</em> is 0 (unlimited). </li> + <li> <strong>l<em>tolerance</em></strong>: next rotations will be triggered +when the size of <tt>current</tt> goes higher than <em>filesize</em> minus +<em>tolerance</em>. <em>tolerance</em> cannot be more than half of +<em>filesize</em>. By default, <em>tolerance</em> is 2000. </li> + <li> <strong>r<em>cooldown</em></strong>: if an error occurs during operations +on the next logdirs, retry every <em>cooldown</em> milliseconds. By default, +<em>cooldown</em> is 2000; it's strongly discouraged to set it to a value +under 50. </li> + <li> <strong>E<em>alertsize</em></strong>: only the first <em>alertsize</em> +bytes of the selected lines will be used in the next alerts. An +<em>alertsize</em> of 0 means no limit. By default, <em>alertsize</em> is +200. </li> + <li> <strong>^<em>statussize</em></strong>: only the first <em>statussize</em> +bytes of the selected lines will be used in the next status file updates. +If a line is shorter than <em>statussize</em> bytes, the status file will be +padded with newlines so it is always <em>statussize</em> bytes long. 0 means +an unpadded, unlimited status file. By default, <em>statussize</em> is 1001. </li> + <li> <strong>!<em>processor</em></strong>: registers +<tt>execlineb -Pc <em>processor</em></tt> as a processor for the next logdirs; +<tt>execlineb</tt> must be found in s6-log's PATH. +If <em>processor</em> is empty, no processor will be set for the next logdirs. +By default, no processor is set. </li> +</ul> + +<h3> Action directives </h3> + +<p> + These directives determine what s6-log actually <em>does</em> with the +selected lines. +</p> + +<ul> + <li> <strong>e</strong>: alert. s6-log will print "s6-log: alert: ", +possibly prepended with a timestamp, followed by the first +<em>alertsize</em> bytes of the line. </li> + <li> <strong>=<em>statusfile</em></strong>: status. s6-log will atomically +update the <em>statusfile</em> file with the first <em>statussize</em> +bytes of the line, and pad it with newlines. s6-log must have the right +to write to <em>statusfile</em> and to <em>statusfile</em>'s directory. </li> + <li> <strong><em>dir</em></strong> (must start with '/' or '.'): logdir. +s6-log will log the line into the logdir <em>dir</em>. s6-log must have +the right to write to <em>dir</em>.</li> +</ul> + + +<h2> Signals </h2> + +<ul> + <li> SIGTERM: If s6-log has been run with the <tt>-p</tt> option, does nothing. +Without this option, SIGTERM instructs s6-log to stop reading stdin after the +next newline and exit after logging the last line. </li> + <li> SIGALRM: triggers a rotation on every logdir s6-log is monitoring, +as if the <tt>current</tt> file in those logdirs had reached the size +limit. </li> +</ul> + +<h2> Examples </h2> + +<pre> + s6-log -bt n20 s1000000 /var/log/services/stuff +</pre> + +<p> + Logs all of stdin, prepending every line with a timestamp, into the +<tt>/var/log/services/stuff</tt> logdir, with a maximum archive of +20 log files of 1 MB each; makes sure every line has been written +before reading the next one. +</p> + +<pre> + s6-log -t n30 E500 - +fatal: e - +^STAT =/var/log/foobard/status f s10000000 S15000000 !"gzip -nq9" /var/log/foobard +</pre> + +<ul> + <li> Sends alerts to stderr with the 500 first bytes of lines containing "fatal:". </li> + <li> Maintains the <tt>/var/log/foobard/status</tt> file at 1001 bytes, +updating it when it finds a log line starting with "STAT". </li> + <li> Logs all other lines to logdir <tt>/var/log/foobard</tt>. When <tt>current</tt> +reaches at least 9998 kB (i.e. 10 MB filesise minus 2kB tolerance), pipe it +through <tt>gzip -nq9</tt> and save the result into a timestamped archive file, with +a maximum of 30 such files or a total of 15 MB of compressed archive files. </li> +</ul> + + +<h2> Why use execlineb to interpret the "processor" string ? </h2> + +<p> + Because it is <em>exactly</em> what +<a href="http://skarnet.org/software/execline/execlineb.html">execlineb</a> +is for. +</p> + +<ul> + <li> Directly executing <em>processor</em> is not flexible enough. We want +to be able to run a complete command line, with an executable name and its +arguments. </li> + <li> We could interpret the <em>processor</em> string via <tt>/bin/sh</tt>. +This is what <a href="http://cr.yp.to/daemontools/multilog.html">multilog</a> +does. However, <tt>/bin/sh</tt>, despite being the traditional Unix interpreter, +is overpowered for this. We don't need a complete shell script interpreter: +most <em>processor</em> commands will be very simple, with only two or three +words, and we only need a way to turn a string into an <em>argv</em>, i.e. a +command line. </li> + <li> <a href="http://www.skarnet.org/software/execline/execlineb.html">execlineb</a> +was designed just for this: to turn simple strings into command lines. +It is a very fast and lightweight script launcher, that does not do any heavy +startup initialization like <tt>/bin/sh</tt> does. It happens to be the perfect +tool for the job. </li> + <li> Yes, I also did this on purpose so people have a reason to use the +<a href="http://www.skarnet.org/software/execline/">execline</a> language. Do not +look at me like that: it <em>really</em> is the perfect tool for the job. </li> +</ul> + +<h2> Why have another logging mechanism ? </h2> + +<p> + Because the syslog mechanism and all its implementations (save one) suck. +I'm not being judgmental; I'm just stating the obvious. +</p> + +<a name="diesyslogdiedie"><h3> The syslog design is flawed from the start </h3></a> + +<p> +<a href="http://blog.gerhards.net/2007/08/why-does-world-need-another-syslogd.html">When +asked why he started rsyslog</a>, Rainer Gerhards came up with a lot of +hand-waving and not a single word about technical points. There is a +reason for that: rsyslog is forked from sysklogd! So, no matter how +many bells and whistles are added to it, it still suffers from the same +basic flaws. +</p> + +<p> + The problem with syslogd does not come from such or such implementation. +The problem comes from syslog's <em>design</em> in the first place. +</p> + +<ul> + <li> syslog makes you send <em>all</em> your logs to the same place. +The logs from a zillion processes are read by a single syslogd server. +The server checks log lines against system-wide regular expressions +to decide where to write them. This raises the following issues: + <ul> + <li> Unless the client explicitly mentions its name in every log +line, there is no way for log readers to know what process generated a +given line. Some syslogd implementations can log the pid of the client; +big deal. </li> + <li> Log lines from every client have to run through the same regular +expression matching. This requires huge regular expression sets, and an +obvious performance impact, to do anything meaningful. And as a matter +of fact, standard syslogd configurations don't do anything meaningful: +they separate the logs into a few streams such as <tt>/var/log/messages</tt>, +<tt>/var/log/auth.log</tt>, <tt>/var/log/daemon.log</tt> or +<tt>/var/log/syslog</tt> with very vague semantics. All of syslogd's +line processing power remains unused, because making real use of it would +be too complex. </li> + </ul> + <li> syslogd logs to <em>files</em>. This is wrong, because files grow +and disks fill up. Sure, there are utilities such as <tt>logrotate</tt> +to perform cleaning up, but as long as logging and log rotation are +kept separate, there is a race condition: a log file can grow and fill +up a disk before a rotation occurs. I am all for separating tasks that +can be separated, but there is no choice here: <em>logging and log +rotation management must be done <strong>by the same tool</strong></em>. +Only the Busybox implementation of syslogd does this, and that is a +feature added by the Busybox developers who are aware of the problem +but want to maintain compatibility with the historical syslogd. +No other syslogd implementation I know of manages its log files: that's a +flaw that no amount of external tools is going to fix. </li> + <li> syslogd is a complex process that runs as root. We all know what +complex processes running as root mean: bugs turning into security holes. </li> + <li> syslog requires a syslogd service, and fails otherwise. A syslogd +service may not be present, it may fail... or it may want to log stuff. +Who's going to take care of syslogd's error messages ? </li> +</ul> + +<p> + syslog is slow, it's unsafe, and it's incomplete. The only reason people +use it is because it's historical, it exists, and there hasn't been any +serious alternative yet, except maybe +<a href="http://cr.yp.to/daemontools/multilog.html">multilog</a>, which +s6-log improves upon. +</p> + +<a name="loggingchain"><h3> A not-so-modest proposal: the logging chain </h3></a> + +<p> + Unix distributions already do this to some extent, but it's at best +unclear where the logs go for any given program. +</p> + +<ul> + <li> Every program, without exception, should send its logs (be it +error messages, warning messages, or anything) to its <em>standard +error descriptor</em>, i.e. fd 2. <em>This is why it's open for.</em> </li> + <li> When process 1 starts, the logging chain is rooted to the +<em>machine console</em>: anything process 1 sends to its stderr +appears, without modification, on the machine console, which should +at any time remain the last resort place where logs are sent. </em> + <li> Process 1 should spawn and supervise a <em>catch-all logging +mechanism</em> that handles logs from every service that does not +take care of its own logging. Error messages from this logging +mechanism naturally go to the machine console. </li> + <li> Process 1's own error messages can go to the machine console, +or <a href="s6-svscan-1.html#log">dirty tricks can be used</a> so they +go to the catch-all logging mechanism. </li> + <li> Services that are spawned by process 1 should come with their +own logger service; the supervision mechanism offered by +<a href="s6-svscan.html">s6-svscan</a> makes it easy. Error messages +from the loggers themselves naturally go to the catch-all +mechanism. </li> + <li> User login mechanisms such as <tt>getty</tt>, <tt>xdm</tt> or +<tt>sshd</tt> are services: they should be started with their own +loggers. Of course, when a user gets a terminal and a shell, the +shell's stderr should be redirected to the terminal: interactive +programs break the automatic logging chain and delegate responsibility +to the user. </li> + <li> A syslogd service <em>may</em> exist, to catch logs sent via +syslog() by legacy programs. But it is a normal service, and logs +caught by this syslogd service are not part of the logging chain. + It is probably overkill to provide the syslogd service with its own +logger; error messages from syslogd can default to the catch-all logger. + The s6 package, including the <a href="ucspilogd.html">ucspilogd</a> program, +combined with the +<a href="http://skarnet.org/software/s6-networking/">s6-networking</a> +package, provides enough tools to easily implement +a complete syslogd system, for a small fraction of the resource needs and +the complexity of native syslogd implementations. </li> +</ul> + +<p> + So, given a program, where are its logs sent ? +</p> + +<ul> + <li> Logs sent via syslog() will be handled by the syslogd service as +usual. Smart administrators will make sure that those ones are as few as +possible. The rest of this analysis is about logs sent to stderr. </li> + <li> If the program is descended from a user's interactive program, +its logs are sent to the user's terminal or the user's choice redirection +target. </li> + <li> If the program is descended from a logged service, its logs are +naturally sent to the service's logger. </li> + <li> Else the logs are sent to the catch-all logger. </li> + <li> Only the catch-all logger's error messages, the kernel's fatal +error messages, and maybe process 1's error messages, are sent to the +system console. </li> +</ul> + +<a name="#howtouse"><h3> What does s6-log have to do with all this ? </h3></a> + +<p> + In a <em>logging chain</em> situation, every service must have +its own logger. To avoid syslogd's design mistakes, one logger process +per service must be run. s6-log fits that role. Using s6-log as +your one-stop logger offers the following benefits: +</p> + +<ul> + <li> Every instance of s6-log can run as a different user, so it's +easy to give different access rights to different logs. It is also +more secure not to have any logger process running as root. </li> + <li> s6-log consumes very little memory per instance (unless it +accumulates unflushed log lines, which you can avoid with the +<tt>-b</tt> option). So, launching a lot of s6-log processes does +not waste resources. </li> + <li> s6-log is vastly configurable via logging scripts; every instance +is as powerful as a traditional syslogd. </li> + <li> s6-log can log to a RAM filesystem and thus is suitable as a +catch-all logger. Clever tricks like Upstart's <em>logd</em> or daemontools' +<a href="http://cr.yp.to/daemontools/readproctitle.html">readproctitle</a> +are just that: tricks. s6-log gives a unified interface to all of +your system's loggers. </li> +</ul> + + +<a name="#network"><h3> You're wrong about being as powerful as +syslogd: s6-log does not do remote logging. </h3></a> + +<p> + You mean you want to send, <em>live</em>, every <em>log line</em> +over the network via <em>UDP</em> ? You can't be serious. +</p> + +<p> + Do yourself a favor and use s6-log to write log lines to a logdir, +with a processor script that sends files-being-archived to the +network, possibly after compressing them. More reliability, less +log lines lost, less network traffic, better engineering. If you +have no disk to even write the <tt>current</tt> files to, write +to a small RAM filesystem. +</p> + +<p> + If you <em>have to</em> log stuff <em>live</em> via the network, you +do not need any local logging software. You don't even need syslogd. +Just filter your stderr via some <tt>grep</tt> that selects lines for +you, then sends them to a network socket. A trivial shell script, or +<a href="http://skarnet.org/software/execline/">execline</a> +script, can do that for you. +</p> + +<p> + Do not insist on using syslogd. It does nothing magical, and nothing +that can't be done in a simpler way using simpler tools. +</p> + +</body> +</html> |